1. Field
This disclosure relates to technology for non-volatile data storage.
2. Description of the Related Art
A variety of materials show reversible resistivity-switching behavior, and as such may be suitable for use as memory elements. One type of material having reversible resistivity-switching behavior is referred to as resistance change memory (ReRAM). Transition metal oxides have been proposed for ReRAM. A second type of material having reversible resistivity-switching behavior is referred to as phase change memory (PCRAM). Chalcogenides, which may change between a crystalline state (conductor) and an amorphous state (insulator), have been proposed for PCRAM. Other materials such as carbon polymers, perovskites, and nitrides have also been proposed as memory elements having reversible resistivity-switching behavior.
Upon application of sufficient voltage, current, or other stimulus, the reversible resistivity-switching material switches to a stable high-resistance state. This resistivity-switching is reversible such that subsequent application of an appropriate voltage, current, or other stimulus can serve to return the reversible resistivity-switching material to a stable low-resistance state. This conversion can be repeated many times. For some switching materials, the initial state is high-resistance rather than low-resistance.
These switching materials are of interest for use in nonvolatile memory arrays. One type of memory array is referred to as a cross-point array, which is a matrix of memory elements typically arranged along x-axes (e.g., word lines) and along y-axes (e.g., bit lines). In some aspects, a digital value is stored as a memory resistance (high or low). The memory state of a memory cell can read by supplying a voltage to the bit line or/and word line connected to the selected memory element. The resistance or memory state can be read as an output voltage or current of the bit line connected to the selected memory cell. One resistance state may correspond to a data “0,” for example, while the other resistance state corresponds to a data “1.” Some switching materials may have more than two stable resistance states.
Non-volatile memories formed from reversible resistivity-switching elements are known. For example, U.S. Patent Application Publication 2006/0250836, filed May 9, 2005 and titled “Rewriteable Memory Cell Comprising A Diode And A Resistivity-Switching Material,” which is hereby incorporated by reference herein in its entirety, describes a rewriteable non-volatile memory cell that includes a diode coupled in series with a reversible resistivity-switching material such as a metal oxide or metal nitride. Such memory cells can be programmed by applying one or more programming signals to cause the reversible resistivity-switching to change from a low resistance state to a high resistance state, which may be referred to as RESETTING the memory cell. Similarly, the memory cells can be programmed by applying one or more programming signals to cause the reversible resistivity-switching to change from the high resistance state to the low resistance state, which may be referred to as SETTING the memory cell.
Both unipolar and bipolar modes of operation of the cross-point memory arrays have been proposed. In bipolar operation, the high resistance state is established by applying a voltage having one polarity and the low resistance state is established by applying a voltage having the opposite polarity. In unipolar operation, the high resistance state and low resistance state are established by applying voltage of the same polarity.
Some memory arrays use a steering device in series with the reversible resistivity-switching element to control the current flow for SET and RESET. That is, with a cross-point memory array some memory cells are selected for programming or reading, whereas many others are unselected and therefore should not be programmed or read during the present operation. The steering element helps to control which memory cells get programmed or read during a given operation. An example of a steering element is a p-i-n diode placed in series with each reversible resistivity-switching element. With appropriate voltages applied to the bit lines and word lines, each memory element can be separately programmed and read. However, with a p-i-n diode typically only unipolar switching is possible. However, unipolar operation may suffer from problems such as requirement of a high RESET current.
One proposal for bipolar operation of cross-point memory arrays is to place a metal/insulator/metal (MIM) diode in series with the resistive memory cell. However, it can be difficult to fabricate MIM diodes having desirable properties such as a sufficiently high forward bias current.